The invention relates to a method and system for heating and deaerating chips, or like comminuted cellulosic fibrous material, prior to digestion thereof.
The need for preheating and deaerating chips have been recognized substantially from the time that the first continuous digesters were constructed. In early designs of hydraulic digester feed systems, a horizontal presteaming vessel with an internal screw for transporting the chips through the vessel was utilized. The inlet of the vessel was equipped with a rotary pocket plug valve which maintained a pressure seal at the inlet, and steam was added to the bottom of the vessel. The vessel was operated at a pressure of about 15-20 psi, providing a temperature of 250.degree. F.+ in the vessel. Air, plus some steam, was stripped from the top of the vessel. Time for the chips to pass through the vessel was in the order of 3-5 minutes.
As digester systems increased in size, the horizontal screw-type steaming vessel reached its practical size limit. Presently, the majority of presteaming of the chips is now done in the chips bin preceding the chips meter and preceding the pressure feeder of the horizontal steaming vessel. A typical system for presteaming chips in the chips bin is shown in U.S. Pat. No. 4,124,440.
With most of the steaming presently being done in the chips bin, the horizontal steaming vessel of modern, large-sized plants, is now used to strip air from the incoming chips, and to maintain an overpressure on the low pressure side of the rotary high pressure feeder.
Unless the air is stripped from the chips before they enter the hydraulically filled digester, the chips will tend to float, and hang-ups of the chip column in the digester will occur. Steam introduced into the chips in the horizontal steaming vessel effects stripping of the air from the chips.
Also, it is necessary that flashing of liquor into steam does not occur in the high pressure feeder, since if it does occur water hammer results in the chip feeder's circulation lines, with many undesirable consequences. The steam introduced in the horizontal steaming vessel thus maintains an overpressure of about 15 lbs./sq.in., which is usually sufficient to prevent flashing in the high pressure feeder.
According to the present invention, heating and deaeration of chips in a paper pulp production process are accomplished in a manner that has a number of advantages over conventional prior art systems. According to the present invention, presteaming of the chips is done in a separate and distinct manner from the stripping of the air from the chips, and deaeration is thus accomplished more efficiently. The practice of the present invention also reduces consumption of low pressure steam, and reduces capital investment for 700+ ton/day systems (which are what are commonly built today), including by eliminating the low pressure feeder which is today commonly employed between the chips bin and the horizontal vessel. Also, the temperature of the slurry of chips in liquor fed by the high pressure feeder to the digester is lower than in conventional systems, providing less possibility of flashing in the high pressure feeder, and the overpressure to prevent flashing is provided hydraulically.
According to one aspect of the present invention, comminuted cellulosic fibrous material is treated, prior to digestion, by heating the material by subjecting it to low pressure steam in a first stage; removing air from the heated material in a manner distinct from the heating, and without introducing additional heat, in a second stage remote from the first stage; and passing the heated, deaerated material to a digesting stage. The deaeration step is practiced by entraining the material in liquid immediately after heating, passing the material entrained in liquid in a predetermined first path; and circulating deaerated liquid in a second path generally transverse to the first path and into contact with the material flowing in the first path, to effect removal of air therefrom. Presteaming in the vertical steaming vessel, and particular flow of fluids in the practice of the method, also provide advantageous results.
According to another aspect of the present invention, a novel structure for feeding steam into a vertical presteaming vessel is provided, and a novel deaeration vessel is provided. The steaming vessel and deaeration vessel are interconnected by a first vertical chute having a chips meter (but no low pressure feeder) therein, and the deaeration vessel is connected by a second, hydraulically filled, chute to a conventional high pressure transfer device which in turn is connected in a conventional way to a continuous digester.
The vertical presteaming vessel comprises: a generally vertical vessel shell; means for introducing steam into the shell to effect steaming of material therein; and the means for introducing steam into the shell comprising: a plurality of uniformly radially spaced nozzles disposed around the periphery of the vertical vessel adjacent the bottom thereof; a centrally extending conduit disposed vertically in the vertical vessel, and including a plurality of pipes therein, each pipe having a steam introducing orifice formed therein at generally the same level as the vertical position of the nozzles, the pipes being generally uniformly radially spaced; and means for feeding steam to the nozzles and the pipes so that steam is introduced into a nozzle at the same time as steam is introduced into a pipe so that the steam flowing through a nozzle flows with generally the same horizontal vector as the steam flowing from a pipe. Means for feeding steam to the nozzles and pipes preferably comprises: a pair of synchronized rotating valve plugs each mounted in a valve housing having a plurality of circumferentially radially spaced discharge ports, and having a steam introduction port; and means for effecting synchronized rotation of the valve plugs in the valve housings, each discharge port from one valve housing operatively connected up to a nozzle, and each discharge port from the other valve housing operatively connected to a steam introduction pipe.
The deaeration means preferably comprises a generally horizontally extending vessel, having a generally horizontal axis, and a rotatable screw extending along the axis. Steamed chips entrained in liquid are fed into the horizontal vessel at one end thereof, and deaerated chips are removed from the vessel at the other end thereof. Liquid circulation loops are provided at both the inlet to and outlet from the horizontal vessel. At a central portion of the vessel, deaerated liquid is continuously circulated into contact with material passing in the vessel, screens generally parallel to the axis of the vessel being part of the system for providing for the flow of deaerated liquid. A closed recirculatory loop of such liquid is provided, and a liquid and air separator is provided in that loop to effect deaeration of the liquid flowing therein.
It is the primary object of the present invention to provide a method and apparatus for effectively treating comminuted cellulosic fibrous material, and the like, to effect heating and/or deaeration thereof. This and other objects of the present invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.